The present invention relates generally to the field of coatings. In particular, the present invention relates to low stress coatings.
Coatings are typically used on gas turbine engine components in order to protect the underlying component from degradation and wear. The coatings, such as abradable outer air seals for fan cases, are typically between approximately 0.15 inches and approximately 0.28 inches thick. At greater thicknesses, the coating may experience excessive tensile and compressive stresses which lead to cracking. Conventional spray technology for applying the coatings use standard plasma spray torches, such as the Sulzer-Metco 3MB, Sulzer-Metco F4, Triplex torches, or other similar designs. However, these spray techniques are designed for maximum particle heating and deposition efficiency. Another spray technique used is high velocity oxygen fuel spray (HVOF). One concern with HVOF for applying thick coatings is that the velocity may be too high, causing excessive compressive stress in the resulting coating.
A concern with current plasma and flame spraying techniques used in the art for applying coatings of this thickness is that they commonly produce a tensile stressed coating. The tensile stresses develop as the powder particles are deposited into the coating and are related to factors including, but not limited to: the kinetic energy of the particles, how much the particles have been melted (herein after referred to as the molten fraction), and the temperature of the component on which the coating is being applied. In addition, if the coating is applied too thickly, the tensile stress, which is inherent in the coating, results in loss of bond strength, cracking, and delamination due to the excess accumulation of tensile stress. The tensile stress may ultimately reduce the durability of the coating to the point where it may spontaneously delaminate during the manufacturing process. In addition, most application processes tend to distort the component on which the coating is applied.
It would thus be beneficial to develop a low tensile-stressed coating and a process of depositing the low tensile-stress coating.